Dipole current lead for a plasma containment device



2 1 9 1 2 1 mmmz @mmm April 29, 1969 HAMMEL ET AL 3,441,775

DIPOLE CURRENT LEAD FOR A PLASMA CONTAINMENT DEVICE Filed Oct. :51, 1966 Sheet of 2 FIELD NULL VEN Jay Ham April 29, 1969 HAMMEL ET AL 3,441,775

DIPOLE CURRENT LEAD FOR A PLASMA CONTAINMENT DEVICE sheet 2 or2 Filed Oct. 31., 1966 H /H iliiw Fig. 2

INVENTOR. Jay E. Hamme/ BY M Q. QM M United States Patent US. Cl. 313-231 2 Claims ABSTRACT OF THE DISCLOSURE A device to provide current feed to an inner conductor of a plasma containment machine without introducing a field null and corresponding plasma leakage. The inner lead of a coaxial cable supplies current to said inner conductor and a conducting arm contacts the outside of the cable shield near but notin contact with said inner conductor thereby terminating the field n-ull before contact with said inner conductor.

The invention described herein was made in the course of, or under, acontract with the US. Atomic Energy Commission.

This invention relates to means for minimizing contamination of plasmas in a confinement geometry utilizing interior conductors.

In the investigation of plasma confinement, geometries with interior conductors have been studied. Initially, interest centered on levitated quadrupoles but it soon become apparent that theresistance of the interior conductor presented a fundamental difiiculty because of the electric field which produces an E x B drift toward the conductor. Calculations on drift to the interior conductor caused by the resistance showed that for long time confinement of kev. deuterons a large device was required and the magnetic field energy needed was impossibly large.

A directly-driven interior conductor does not have this difliculty but does substitute the prohibitive feature of a current feed which must cross the plasma confinement region.

Linear dipole guards have been proposed for the current lead to an inner conductor in a rotating plasma machine. This arrangement has the advantage of preventing bombardment of the current-carrying leads. The inventor has determined that one problem which would be encountered in such a device would be the field null created by the dipole protection. Each null intersects the interior conductor and allows plasma leakage to the interior conductor. This plasma leaking toward the interior conductor will bombard the conductor and cause contamination which will be intolerable for long-time confinement of plasma.

According to the present invention, the guard terminates before it reaches the interior conductor so that the null does not reach its surface. Thus, the only leak along the field null will be at the outer boundary where the plasma can be dumped outside the machine. This direct leak toward the outside will guarantee a low density in the null. The guard is thus a hairpin which terminates slightly before it reaches the inner conductor.

The object of the present invention is the long-term confinement of a plasma in a geometry utilizing an interior conductor.

FIGURE 1 is a diagrammatic sectional view of the conductors showing the means of supporting and feeding current to the interior conductor.

FIGURE 2 is a detailed sectional view of the guarded lead and support of the present invention.

FIGURE 3 is a diagrammatic sectional view taken at A--A of FIG. 2 showing the field configuration with the currents at a value which makes the leads force free.

Referring first to FIG. 1 of the drawings, there will be seen to be two circular interior conductors 1. Passage of current through these conductors sets up a magnetic field within the space 4 bounded by outer conductor 2 and an equal (but opposite) current in cylindrical outer conductor 2. Gun 3, which may be of the coaxial type, injects plasma into space 4. The current is introduced to interior conductors 1 through guarded leads 5 (which also serve as supports). Circular holes 6 and 7 are provided in outer conductor 2 for guarded leads and plasma from gun 3, respectively. All equipment shown in FIG. 2 is contained in a vacuum tank (not shown).

FIGURE 2 shows a guarded lead 5 schematically (with auxiliary circuitry) in more detail. Capacitor 8 supplies current to interior conductor -1 through inner lead 9 of coaxial cable 15 when switch 10 is closed. This current returns on the inside of shield 14 of coaxial cable 15. Switch 11 is closed at the same time as switch 10, thereby discharging capacitor 12. Current fiows down arm 13 and up the outside of shield 14 of coaxial cable 15. Since current flows down arm 13 and up the outside of shield 14, the device retains the advantage of the linear dipole (the induced magnetic fields prevent bombardment of the leads). However, since the dipole is terminated by connection of arm '13 to shield 14 at point 16, the nulls 18 will turn around as at 17 before reaching any solid material. Accordingly, no contamination will be found in the system. A small leak out of the annulus 19 will occur. This can be made small since the null is only one ion gyro radius and the device can be made arbitrarily large.

In the preferred embodiment interior conductor 1 is a three-turn split ring of 24 cm. diameter made of dural and carrying 3 megamps. Shield 14 is 1 /3 inch diameter Be-Cu. The dielectric of the cable is epoxy and fiberglass. Arm 13 is also 1 /8 inch OD. and makes a good electrical connection (e.g.,' welding) with shield 14. Capacitors 8 and 12 are banks delivering 10 kv., 1.4 megajoules and 10 kv., kilojoules, respectively. Switches 10 and 11 are sets of type A ignitrons. The gun injects 10 total deuterons at energies of 2.5 to 5 kev. energy in 2 ,usec. For this test of the dipole protector the plasma will have a decay time of 500 ,usec. out through the nulls of the system.

The plasma flows along the center of annulus 19 and crook 20 of arm 13 and outer conductor 2 should be at least 5 Larrnor radii distant from this plasma path. In order to make the leads force free a guard current of about 1.2 10 amperes and a separation between 13 and 14 of two inches are used. The outer conductor should be about one inch thick for strength since the generated magnetic field is quite strong (45 kilogauss in the preferred embodiment).

FIGURE 3 shows the magnetic fields created by the conductors at a section taken at A-A of FIG. 2. In arm 13 current is flowing into the paper and on shield 14 it is flowing out of the paper. This figure is applicable both to the folded dipole of the present invention and the linear dipole. However, the shown nulls terminate before reaching a solid surface in the present invention and this improvement allows the use of the desirable interior conductor geometry.

What is claimed is:

1. A current lead and support for the interior conductor of a plasma confinement geometry wherein one lead is a coaxial cable and a second is a conductor parallel to the said coaxial cable except that said second lead is folded near but not in contact with said interior conductor such that the second lead is in good electrical contact with the shield of the coaxial cable.

2. A current lead and support as in claim 1 wherein separate current sources supply the said first and second leads.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner.

E. R. LA ROCHE, Assistant Examiner.

US. Cl. X.R. 

